Tin alloy solder compositions

ABSTRACT

A lead-free and bismuth-free solder alloy composition for electronic assembly applications having reduced toxicity. The alloy composition comprises about 0.01% to about 4.5% silver; about 0.01% to about 3% copper; about 0.002% to about 5.0% antimony; about 85% to about 99% tin and about 0.002% to about 1% of either nickel or cobalt. The alloy composition has a melting temperature of about 217° C., with superior wetting and mechanical strength making the alloy composition well suited for electronic circuit board manufacture and lead less component bumping or column arrays, and replacement of conventional tin-lead solders.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to US Provisional Patent Application No. 60/679,869, filed on May 11, 2005.

FIELD OF THE INVENTION

The invention relates to a lead-free and bismuth-free tin alloy that contains antimony and nickel or cobalt.

BACKGROUND OF THE INVENTION

The present invention relates generally to an improved solder composition. More specifically, the present invention relates to an improved solder composition that contains no lead or bismuth yet still achieves superior soldering characteristics.

In the electronic manufacturing of printed circuit boards and the assembly of components thereon, the solders employed generally contain tin and lead to provide mechanical and electrical connections. Solders that contain tin and lead typically yield highly reliable connections in both automated and manual soldering and provide a surface on printed circuit boards extremely conducive to soldering.

Tin-lead alloys of, for example, sixty (60%) percent tin, forty (40%) percent lead; and sixty-three (63%) percent tin, thirty-seven (37%) percent lead have historically been used for most electronic soldering operations. These alloys have been selected and are preferred because of their low melting temperatures, mechanical strength, low relative cost, as well as superior wetting characteristics and electrical conductivity.

However, the use of such tin-lead solders in the manufacture of printed circuit boards and assembly of components is becoming more and more problematic due to the toxic effects of lead exposure to workers and the inevitable generation of hazardous waste. For example, even small amounts of lead can affect the neurological development of fetuses in pregnant workers. Due to these environmental concerns, action is being taken to limit the amount of lead entering into the environment. Federal and many state government agencies have begun to urge the electronics industry to find alternatives to tin-lead solders to reduce worker lead exposure and lessen the amount of lead waste going back into the environment.

Due to the materials used, many components and printed circuit boards are easily damaged by exposure to high temperatures during manufacture or assembly. Because of heat transfer and distribution limitations and concerns, printed circuit boards are typically exposed to temperatures higher than the liquidus temperature of the alloy employed. In response to this concern, electronic manufacturers are exploring alternative alloys to replace the tin-lead alloys.

The prior art has not provided a solder composition exhibiting optimum wetting and flow properties without toxicity. Currently federal, military and commercial solder specifications lack a suitable non-toxic composition. The following prior art patents illustrate inadequate attempts to meet these needs.

Soviet Union Patent No.183,037, issued to A. I. Gubin et al. discloses an alloy containing antimony of 1±0.3%; copper 2±0.3%; silver 5±0.3% and the remainder being tin and having a melting point of 225°-250° C. This alloy has a liquidus temperature that does not allow it to be used in electronic soldering because the soldering temperature required to flow the alloy would destroy the printed circuit board and many of the components. No feasible equipment or means currently exists to allow this alloy to be used for the purpose of electronic soldering or coating. Due to the high silver content, this alloy has an economic disadvantage in the marketplace.

U.S. Pat. No. 3,503,721, issued to Lupfer, discloses a tin-silver alloy of 96.5% tin and 3.5±0.5% silver with wetting and electrical conductivity characteristics marginally acceptable to suit the needs of the electronics industry. However, this alloy has mechanical strength weaknesses that would prohibit its use on a wide range of electronic printed circuit board assemblies. For example, creep strength, a measure of flow under pressure, and percent elongation, metal stretching before fracture, are considerably lower than that of the tin-lead alloys now used. Even with the common tin-lead alloys, solderjoints stress fractures are the cause of many field failures in printed circuit boards where vibration or temperature variations occur. In addition, the liquidus temperature of 221 ° C. requires that automated soldering be accomplished at a temperature that in many situations would damage the printed circuit board and/or the components. Due to the high content of silver, the cost of this alloy is considerably higher than tin-lead alloys. For each percentage point of silver added to the alloy, the price increases by approximately $0.75/lbs. (based on a silver market of $5.00/troy ounce).

U.S. Pat. No. 4,778,733, issued to Lubrano et al. discloses an alloy containing, by weight, 0.7% to 6% copper; 0.05% to 3% silver; with the remainder being tin with a temperature range of 440°-630° F. This alloy has a melting temperature that is too high to be used in a wide range of electronic soldering applications without damaging printed circuit boards or components. In addition, the alloy disclosed by Lubrano et aL. exhibits inferior soldering performance, slow wetting times and mechanical strengths ill-suited to electronic assembly applications.

U.S. Pat. No. 4,695,428, issued to Ballentine et al. discloses an alloy containing 0.5-4% antimony; 0.5-4% zinc; 0.1-3% silver; 0.1-2% copper; 88-98.8% tin. The zinc content in this alloy causes the alloy to oxidize quickly. This inhibits wetting and flow, producing high dross formation which results in extremely high defect levels. The productivity lost in using such a composition for mass electronic soldering makes it an unacceptable alternative to tin-lead solders.

U.S. Pat. No. 4,758,407, issued to Ballentine et al. discloses an alloy containing tin, copper, nickel, silver and antimony. All of the alloy combinations disclosed by Ballentine et al. have liquidus temperatures in excess of those required for electronic assembly. The lowest disclosed liquidus temperature is 238° C., which is unacceptable for use in the electronics industry.

The most commonly used lead-free alloy is comprised of tin-silver-copper. Industry testing has proven that tin-silver-copper, lead-free solder alloys do not offer sufficient drop testing characteristics as compared to tin-lead solder alloys, especially on 0.3 mm BGA devices. Common tin-silver-copper alloys, known as SAC alloys, contain 3-4% silver and 0.5-1% copper. The main problem with these alloys in a BGA type application is the AgSn intermetallic plate formation as well as Kirkendal voiding that occurs. To make SAC alloys more stable, several elements have been added to reduce copper erosion as well as limit large intermetallic plates from forming. For example, P, Ge, rare earth metals, Sb, Ni, and Co have been tried. In addition, the solder alloys composed of tin-silver-copper-antimony described in U.S. Pat. Nos. 5,352,407 and 5,405,577, issued to Seelig et al. show improvement versus tin-silver-copper alloys. However, this alloy shows some improvements of tin, silver, copper alloys; however there is still a need for enhanced performance.

Since heretofore no acceptable substitute for tin-lead alloys in BGA applications have been found, there remains a need in the electronics industry for an alloy composition without lead or bismuth that can achieve the physical characteristics and application performance of tin-lead solder alloys but without the toxic elements.

SUMMARY OF THE INVENTION

The present invention provides solder alloys with new advantages not found in currently available solder compositions, and overcomes many of the disadvantages of currently available compositions.

The invention is generally directed to novel and unique solder compositions with particular application in the electronic manufacturing of printed circuit boards and the assembly of components therein, as well as lead less component bumping arrays and column arrays. The solder compositions of the present invention achieve desired physical characteristics, such as wetting, peel strength, low melting point, physical strength, fatigue resistance, electrical conductivity, matrix stability, and uniform joint strength, but without the toxic elements found in known tin-lead solder alloys.

The alloy compositions of the present invention include a combination of tin, silver, copper, antimony, and either nickel or copper, to offer a unique set of physical characteristics that allow it to be used as a viable alternative to tin-lead alloys in electronic soldering and printed circuit board coating, as well as lead less component bumping arrays and column arrays. The alloy of the present invention possesses physical characteristics that result in a stronger mechanical joint with superior fatigue resistance to tin-lead alloys, tin-silver alloys, or alloys containing bismuth. In addition, the melting point temperature is lower than any other lead-free or bismuth free alternative solder alloy.

The preferred embodiment of the present invention has a reduced toxicity and a melting point of about 217° C and consisting of, in weight percent, 85-99% tin; 0.01-4.5% silver; 0.01-3.0% copper; and 0.002-5.0% antimony and either 0.0001-1.0% nickel or 0.0001-1.0% cobalt.

It is therefore an object of the present invention to provide solder compositions that are a viable substitute for tin-lead solder alloys.

Another object of the present invention is to provide solder alloy compositions that are well suited for the electronic manufacturing of printed circuit boards and the assembly of components thereon.

It is a further object of the present invention to provide solder alloy compositions acceptable for the electronics industry that contain no lead or bismuth.

It is yet a further object of the present invention to provide solder alloy compositions that are free of toxic elements and safe for the environment.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention are lead-free and bismuth-free solder compositions that contain tin, silver, copper, antimony, and nickel or contain tin, silver, copper, antimony, and cobalt. The solder alloy compositions of the present invention have the physical characteristics and the application performance to economically meet the needs of the electronic industry and the assembly and coating of printed circuit boards. In particular, the alloy exhibits ideal physical characteristics yet does not contain toxic elements as alloys found in the prior art which could harm workers and the environment.

The alloys of the present invention have advantages over the tin, silver, copper, antimony alloy described in the prior art. Below is an independent comparison test between a tin, silver, copper, antimony alloy, as described in U.S. Pat. No. 5,405,577 and a prior art tin-lead solder alloy containing 63% tin and 37% lead. As seen below, the mechanical strength of this alloy is superior to known tin-lead alloys. Tin/Silver/Copper/ 63% Tin/37% Lead Antimony Alloy Tensile Categories UTS(ksi) 4.92 5.73 YIELD STGTH.(ksi) 4.38 4.86 YOUNGS' MODULUS(msi) 52.8 42.40 Compression Categories ELASTIC MOD.(msi) 3.99 4.26 YS(ksi) 4.52 4.33 STRESS 25% (ksi) 7.17 8.54 Hardness Category ROCKWELL 15 W 10.08 18.28

The alloy compositions of the present invention that exhibit the desired physical characteristics is comprised by weight as follows: Metal % Composition Tin (Sn)  85-99% Silver (Ag) 0.01-4.5% Copper (Cu) 0.01-3.0% Antimony (Sb) 0.002-5.0%  Nickel (Ni) or Cobalt (Co) 0.0001-1%   

In an embodiment, the solder composition comprises about 1.75% to about 2.0% silver; about 0.8% copper; about 0.5% antimony; about 0.08% nickel; and about 96.6% to about 96.9% tin. The melting point temperature of the composition is in the range of about 217° C. The liquidus temperature of about 217° C. coupled with superior wetting allows the alloy of the present invention to be used with existing mass and hand soldering equipment without damaging most printed circuit boards or electronic components. In addition, this alloy when tested in JEDEC drop tests of 1500 g ×0.5 meters demonstrated twice the fatigue life of the known SAC alloys. By comparison, an SAC alloy containing antimony but lacking nickel (e.g., having the composition about 1.75% to about 2.0% silver; about 0.8% copper; about 0.5% antimony; and about 96.6% to about 96.9% tin) demonstrated only a 30% increase in fatigue life.

In another embodiment, the solder composition comprises about 0.5% to about 1.75% silver; about 0% to about 0.5% copper; about 0.002% to about 0.2% antimony; about 0.08% to about 0.04% nickel; and about 97.5% to about 99.4% tin.

In another embodiment, the solder composition comprises about 1.0% to about 1.75% silver; about 0.8% copper; about 1.0% antimony; about 0.008% cobalt; and about 96.44% to about 97.2% tin. This alloy also showed an improved fatigue life over SAC alloys in JEDEC drop tests of 1500 g ×0.5 meters.

In another embodiment, the solder composition comprises about 0.02% to about 1.0% silver; about 0.2% to about 0.8% copper; about 0.2% to about 0.8% antimony, about 0.008% to about 0.4% cobalt, and about 97% to about 99.6% tin.

The present solder compositions may comprise about 85% to about 87% tin; about 87% to about 89% tin; about 89% to about 91% tin; about 91% to about 93% tin; about 93% to about 95% tin; about 95% to about 97% tin; or about 97% to about 99% tin, or a combination of two or more of the above ranges (e.g., from about 95% to about 99% tin).

The present solder compositions may comprise about 0.01% to about 0.05% silver; about 0.05% to about 0.1% silver; about 0.1% to about 0.5% silver; about 0.5% to about 1.0% silver; about 1.0% to about 2.0% silver; about 2.0% to about 3.0% silver; about 3.0% to about 4.0% silver; or about 4.0% to about 4.5% silver, or a combination of two or more of the above ranges (e.g., from about 1.0% to about 3% silver).

The present solder compositions may comprise about 0.01% to about 0.05% copper; about 0.05% to about 0.1% copper; about 0.1% to about 0.5% copper; about 0.5% to about 1.0% copper; about 1.0% to about 2.0% copper; or about 2.0% to about 3.0% copper, or a combination of two or more of the above ranges (e.g., from about 0. 1% to about 1% copper).

The present solder compositions may comprise about 0.002% to about 0.005% antimony; about 0.005% to about 0.01% antimony; about 0.01% to about 0.05% antimony; about 0.05% to about 0.1% antimony; about 0.1% to about 0.5% antimony; or about 0.5% to about 1.0% antimony; about 1.0% to about 2.0% antimony; about 2.0% to about 5.0% antimony; or a combination of two or more of the above ranges (e.g., from about 0.1% to about 1% antimony).

The present solder compositions may comprise about 0.002% to about 0.005% nickel; about 0.005% to about 0.01% nickel; about 0.01% to about 0.05% nickel; about 0.05% to about 0.1% nickel; about 0.1% to about 0.5% nickel; or about 0.5% to about 1.0% nickel, or a combination of two or more of the above ranges (e.g., from about 0.01% to about 0.1% nickel).

The present solder compositions may comprise about 0.002% to about 0.005% cobalt; about 0.005% to about 0.01% cobalt; about 0.01% to about 0.05% cobalt; about 0.05% to about 0.1% cobalt; about 0.1% to about 0.5% cobalt; or about 0.5% to about 1.0% cobalt, or a combination of two or more of the above ranges (e.g., from about 0.01% to about 0.1% cobalt).

Not to be limited to any particular theory, the combination of antimony with nickel or cobalt may inhibit the SAC alloy from dissolving copper and forming large intermetallic platelets, thereby yielding a more stable matrix over time, and providing better stability and more uniform joint strength.

The alloys of the invention exhibit excellent wetting and melting temperatures, as well as superior physical strength, electrical conductivity, and thermocycling fatigue, for example. As a result of these excellent physical characteristics, the solder alloy compositions of the present invention may be successfully substituted for the known tin-lead alloys currently used for electronics assembly and printed circuit board manufacture, as well as lead less component bumping arrays and column arrays. Most capital equipment used in electronic soldering can employ these compositions. The low melting temperature is low enough not to cause heating damage to the board or components therein.

The alloy compositions of the present invention are well suited for many different applications. The alloys may be employed in the coating of circuit boards and printed circuit board manufacture by use of “hot-air leveling” or “roll-tinning”. These processes improve solderability on the circuit board. Also, the alloys may be used in the assembly of electronic components on printed circuit boards when using a wavesoldering machine. The alloys are also well suited for formation into various shapes and sizes, such as bars, ingots, wire, chips, ribbons, powder, preform and can be used with a core of flux. Therefore, the alloys of the present invention may be used for assembly of electronic components using solder wire and a heating device to hand solder the components to the board.

In the application of coating printed circuit boards, the compositions of the present invention have superior wetting characteristics and improved productivity. Tin-lead alloys of the prior art are easily contaminated by copper from the PC boards that are dipped into a bath during processing. Since the compositions of the present invention contain copper, minor increases in the copper content do not readily affect performance of the compositions. In addition, these new compositions will not absorb copper as quickly as prior art tin-lead solders. As a result, these new alloys can remain functional much longer than prior art tin-lead alloys to reduce overall solder consumption drastically and reduce outlay by manufacturers. Moreover, the solderability of the coated board is extended because the intermetallics are distributed evenly throughout the grain boundary of the composition. The result is a higher quality printed circuit board that cannot be achieved by the use of prior art solder compositions.

In surfacemount assembly or wavesoldering of components to printed circuit boards, the compositions of the present invention can employ the same hot temperatures, pre-heat temperatures, and process parameters as prior art tin-lead solders now currently in use. The nominal composition is very close to a eutectic alloy which exhibits physical characteristics important to high speed, low defect soldering. Since the solder alloys of the invention are less easily contaminated than tin-lead alloys, an increased usable life of the solder bath results. Further, solder joints formed by wavesoldering yield higher joint strengths and excellent electrical conductivity with even distribution of intermetallics throughout the solder joint.

The solder alloy compositions of the present invention may also be used in the assembly of electronic components using solder wire in a heating device to hand solder the components to the board. Such a method requires a composition that wets and spreads quickly at about 235° to about 260° C. The composition of the present invention can be easily formed into a cored wire solder and used easily and successfully in hand soldering.

Overall, the alloy compositions of the present invention enjoy a combination of a sufficiently low melting temperature for electronic applications, superior wetting characteristics, and superior mechanical strength to make it an excellent alternative to tin-lead alloys for the needs of the electronic industry for manufacture of printed circuit boards and the assembly of components onto the boards. The superior solderability and wetting characteristics yield even pad thicknesses and low copper solubility to provide a tremendous advantage in the solder coating of printed circuit boards, such as by hot air leveling.

INCORPORATION BY REFERENCE

The contents of all cited references (including literature references, patents, patent applications, and websites) that may be cited throughout this application are hereby expressly incorporated by reference. The practice of the present invention will employ, unless otherwise indicated, techniques for the production and use of alloys, which are well known in the art.

EQUIVALENTS

It will be appreciated by those skilled in the art that various changes and modifications can be made to the disclosed embodiments without departing from the spirit or essential characteristics thereof. All such modifications and changes are intended to be covered by the appended claims. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced herein. 

1. A lead-free, bismuth-free solder alloy composition comprising about 0.01% to about 4.5% silver; about 0.01% to about 3% copper; about 0.002% to about 5.0% antimony; about 0.002% to about 1% nickel; and about 85% to about 99% tin.
 2. A lead-free bismuth-free solder alloy composition comprising about 1.75% to about 2.0% silver; about 0.05% to about 0.09% copper; about 0.02% to about 2% antimony; about 0.008% to about 1.5% nickel; and about 94.4% to about 98.2% tin.
 3. A lead-free, bismuth-free solder alloy composition comprising about 1.75% to about 2.0% silver; about 0.8% copper; about 0.5% antimony; about 0.08% nickel; and about 96.6% to about 96.9% tin.
 4. A lead-free bismuth-free solder alloy composition comprising about 0.5% to about 1.75% silver; about 0% to about 0.5% copper; about 0.002% to about 0.2% antimony; about 0.08% to about 0.04% nickel; and about 97.5% to about 99.4% tin.
 5. A lead-free, bismuth-free solder alloy composition comprising about 0.01% to about 4.5% silver; about 0.01% to about 3% copper; about 0.002% to about 5.0% antimony; about 0.002% to about 1% cobalt; and about 85% to about 99% tin.
 6. A lead-free, bismuth-free solder alloy composition comprising about 1.0% to about 1.75% silver; about 0.2% to about .99% copper; about 0.0001% to about 2.0% antimony; about 0.0002% to about 1% cobalt; and about 94.3% to about 98.8% tin.
 7. A lead-free bismuth-free solder alloy composition comprising about 1.0% to about 1.75% silver; about 0.8% copper; about 1.0% antimony; about 0.008% cobalt; and about 96.44% to about 97.2% tin.
 8. A lead-free bismuth-free solder alloy composition comprising about 0.02% to about 1.0% silver; about 0.2% to about 0.8% copper; about 0.2% to about 0.8% antimony, about 0.008% to about 0.4% cobalt, and about 97% to about 99.6% tin.
 9. A lead less component bumping or column array comprising the alloy composition of any one of claims 1-2 and 5-6.
 10. An electronic assembly comprising the alloy composition of any one of claims 1-2 and 5-6.
 11. The alloy composition of claim 1 or 5, wherein a flux core is inserted into the composition to form an electronic assembly flux cored wire solder.
 12. The alloy composition of claim 1 or 5, wherein the composition constitutes a fluxed core of flux and the alloy particles.
 13. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder bar; said solder bar being used in electronic assembly solder machines.
 14. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder ingot, said solder ingot being used in electronic assembly.
 15. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder wire, said solder wire being used in electronic assembly.
 16. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder chip, said solder chip being used in electronic assembly.
 17. The alloy composition of claim 1 or 5, wherein said alloy composition is formed Into a solder ribbon, said solder ribbon being used in electronic assembly.
 18. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder powder, said solder powder being used in electronic assembly.
 19. The alloy composition of claim 1 or 5, wherein said alloy composition is formed into a solder preform, said solder preform being used in electronic assembly.
 20. The alloy composition of claim 1 or 5, wherein said alloy is employed in hot air levelling of printed circuit boards.
 21. The alloy composition of claim 1 or 5, wherein said alloy is employed in assembling surface mounted printed circuit boards.
 22. The alloy composition of claim 1 or 5, wherein said alloy is employed in the solder coating of printed circuit boards.
 23. The alloy composition of claim 1 or 5, wherein said alloy is employed in roll tinning of circuit boards.
 24. The alloy composition of claim 1 or 5, wherein said alloy is employed in surface mount assembly of electronic components onto a printed circuit board.
 25. The alloy composition of claim 19, wherein said solder preform is fluxed.
 26. The alloy composition of claim 19, wherein said solder preform is unfluxed. 